1. Field of the Invention
The field of invention relates to the production of alpha-olefins. More specifically, the field relates to the production of alpha-olefins, especially 1-butene, via oligomerization of ethylene.
2. Description of the Related Art
1-Butene is an important petrochemical, especially for the productions of polyethylene. The reaction of ethylene and other alpha-olefins, especially 1-butene, forms various grades of linear low density polyethylene (LLDPE), a useful commercial polymer.
A source of 1-butene is the butene fraction from the effluent of a fluidized catalytic cracker. The process for recovering 1-butene from the effluent of a FCC requires several difficult process steps that make the process undesirable.
Several commercial processes dimerize ethylene into 1-butene. A commercially successful dimerization process is the Alphabutol™ Process, developed by the Intitute Francais du Petrole (IFP), described in A. Forestiere, et al., “Oligomerization of Monoolefins by Homogenous Catalysts”, Oil & Science and Technology—Review de l'Institute Francais du Petrole, pgs. 663-664 (Vol. 64, No. 6, November 2009). The process appears to use a loop reactor that contains 1-butene as a process fluid to oligomerize ethylene into 1-butene and other higher-carbon count alpha-olefins.
There is one known problem with loop dimerization systems: polymer and oligomer fouling. Long residence times and poor heat removal from the highly exothermic reactions lead to the formation of polyethylene-based residues. Even with a highly selective oligomerization catalyst, free radical initiators from feed impurities and catalytically active debris (for example, rust) can initiate polymerization of ethylene and other alpha-olefins.
A side effect of chronic fouling is increasingly frequent process shutdowns and higher maintenance costs for removing adhered polymer residues. Polymer builds layer upon layer and eventually closes off openings and ports in locations with low fluid flow rates. A polymer coating along the wall of the reactor acts as an insulator, which negatively affects heat transfer. Polymer can also collect debris that can be catalytically active or that can poison the reaction process.
An especially troublesome issue is the formation of “hot spots”. A hot spot is an area where external cooling is ineffective and catalyst activity is high. It represents a loss of process control. A hot spot can be an area of collected polymer that includes catalytically active material that fosters side-reactions, including polymerization. If left unchecked, the hot spot can eventually lead to a process shutdown due to the loss of cooling capacity or a runaway polymerization reaction.
A catalyst inhibitor can halt the undesirable oligomerization and polymerization reaction in the loop reactor effluent. Applying a catalyst inhibitor inside a loop reactor, however, interferes with the desirable oligomerization reaction as it re-circulates back towards the catalyst introduction area. Adding too much inhibitor or allowing it to build up slowly in the recycle loop can kill the oligomerization reaction completely.
To avoid polymer fouling issues, operators of loop-style reactors operate at reduced temperatures or production rates versus optimal production levels or process conditions. Yet, at lower production rates, polymers can build up in stagnant zones of the loop reaction system. Ineffective mixing at lower temperatures and production rates leads to wasted reactants and higher inefficiency. Poor fluid heat transfer at lower production rates can exacerbate the operational problems the operator is attempting to avoid. Lower operating temperature can result in a reduced effective operating window, which makes overall operational control more difficult.